Wondering if you've kept Achilles or Clown tangs before? I've never heard of anyone keeping Achilles successfully over a year without them dieing, unless they're kept in a large school like they're found naturally. I believe Atlantis has some, but there are like 10 of them. They're very gregarious in nature and would only be alone if sick or injured, so they will perish if alone as they are constantly stressed if kept singly. Clowns Tangs are difficult as well but far from impossible. Have to get one that eats first, then you have a decent chance.
Love the tank! I am building a similar dimensioned plywood tank -that is taking forever... I'll be following along.